This invention relates generally to reactor control blades for controlling the power of a light-water nuclear reactor such as a boiling water reactor and relates more particularly to a high-reactivity-worth, long-lived nuclear reactor control blade designed to increase the reactor shut-down margin and extend the lifetime.
In general, conventional boiling water reactor control blades have a construction such that a multiplicity of neutron absorbing rods are inserted in a plurality of wings formed of elongated U-shaped sheaths fixed to a central tie rod. Each of the neutron absorbing rods is constituted by, for example, a stainless steel cover tube filled with boron carbide (B.sub.4 C) grains provided as a neutron absorber.
While this reactor control blade is inserted in a core section of a nuclear reactor such as a boiling water reactor, the neutron absorber which fills the sheaths is irradiated with neutrons and gradually loses neutron absorbing ability. The nuclear reactor control blade is therefore changed after being used for a predetermined operation period.
Each wing of the control blade used in the core section of a nuclear reactor is not irradiated with neutrons uniformly over the entire area. An insertion end region and an outer edge region of each wing, for example, are irradiated with neutrons more intensely. Part of the neutron absorber which fills each of those region therefore absorbs neutrons at a higher rate, becomes worn faster and reaches the nuclear lifetime faster. Consequently, the whole of the reactor control blade must be scrapped even though the nuclear lifetime of the rest of the neutron absorber in the other region is sufficient. The conventional control blade is thus disadvantageous in terms of economy. In addition, an increase in the frequency of replacement of the reactor control blades means an increase in the total period of time taken for replacement operations, resulting in a reduction in the plant factor and, hence, a considerable economical demerit. There is also a risk of increasing the rate at which operators are exposed to radiation.
To prevent this problem and risk, the inventors of the present invention have already proposed a type of nuclear reactor control blade in which a neutron absorber having a comparatively long lifetime such as hafnium is provided in some sections of the control blade where the intensity of neutron irradiation is high.
This reactor control blade has, as disclosed in Japanese Patent Laid-Open No. 53-74697, a hybrid structure in which a long-lived neutron absorber is provided in top end portions and blade edge portions of the wings. This hybrid type of reactor control blade has a lifetime twice as long as that of an ordinary type of control blade.
In the conventional reactor control blades, the wing is filled with a neutron absorber with a density distribution uniform over the entire region of the wing, and sections of the wing divided in the axial direction are equalized with respect to neutron absorbing ability or reactivity. This arrangement, however, allows a certain dispersion of reactivity with passage of time owing to non-uniformity of the neutron irradiation rate such as mentioned above. There is therefore a possibility of a local deterioration in terms of reactor shut-down margin at the last stage of the operating cycle of the reactor.
The reactor shut-down margin distribution (or subcriticality) in the axial direction in the case of operation of the reactor for a predetermined period of time using the above-described type of reactor control blade varies slightly depending upon the design specification of the fuel assembly or the method of operating the reactor, but this distribution is always substantially the same. That is, the reactor shut-down margin is high with respect to the upper and lower ends of the core and is minimum with respect to a position slightly lower than the upper end.
This phenomenon can be explained by the following reason.
If the effective axial length of the reactor core is L, the void coefficient during operation is particularly high in a section close to the upper end of the core ranging between this upper end and a position at a distance of 3/4.L from the lower end of the core. In this section, the power density of the reactor is relatively low and the amount of remaining uranium of a mass number of 235 (U-235) which is a fissile material is comparatively large Neutron spectrum hardening takes place by the effect of generating voids. As a result, the plutonium generation reaction (neutron absorption reaction) is promoted. For this reason, the enrichment of fissile materials in an upper section of the core becomes relatively high after the operation of the reactor, so that the reactor shut-down margin becomes relatively reduced with respect that region.
In the present circumstances, the nuclear fuel burn-up extension and the operating cycle extension will inevitably be promoted because of demands for improvements in the reactor in terms of operation economy. To satisfy such demands, fuels of high enrichment factors are increasingly adopted and, correspondingly, reactor control blades having a long nuclear lifetime and improved in the reactor shut-down margin are urgently required.
If the conventional reactor control blades are applied to a reactor loaded with a nuclear fuel of a high enrichment factor, the reactor shut-down margin becomes relatively reduced and it is necessary to periodically replace the reactor control blades with a short operating cycle. To replace the reactor control blades, it is necessary to perform complicated operations of shutting down the reactor and preliminarily removing from the core a multiplicity of fuel assemblies disposed around the control blades which are to be replaced. The period of time during which the reactor is shut down is thereby extended, resulting in a considerable reduction in the operation efficiency of the reactor and, hence, a deterioration in terms of economy. There is also a possibility of a considerable increase in the amount of working for management.
To satisfy demands for extension of the lifetime of control blades, the applicant of the present invention has developed a long-lived reactor control blade greatly improved. As disclosed in Japanese Patent Laid-Open No. 58-55887, this reactor control blade is constituted by inserting neutron absorbing plates formed from a long-lived neutron absorbing material, e.g., hafnium in wings formed from stainless steel. As a result of the use of long-lived neutron absorbing plates formed from hafnium or the like, the lifetime of the control blade has been increased to a large extent.
This reactor control blade, however, is considerably heavy and expensive as a whole since it makes use of hafnium in the form of a plate which is more expensive than ordinary neutron absorbers and which also has a high density. This control blade cannot be applied to units using conventional control blade driving mechanisms without condition since the design of the mechanism for driving this control blade must be changed to enable the mechanism to withstand the heavy load.